The present invention relates generally to filtration systems and methods for removing particulate matter and/or gaseous pollutants from a stream of gas or other fluid. More specifically, but not by way of limitation, the present invention relates to a more cost effective and/or efficient filtration system and method that reduces the use of sorbents and take advantage of other economic benefits.
Fabric filtration is a common technique for separating out particulate matter in a gas stream. In an industrial setting, fabric filtration is often accomplished in a device known as a baghouse. Generally, a baghouse includes a housing that has an inlet for receiving dirty, particulate-laden gas and an outlet through which clean gas leaves the baghouse. The interior of the housing is divided by a tube sheet into a dirty gas or upstream plenum and a clean gas or downstream plenum, with the dirty gas plenum in fluid communication with the inlet and the clean gas plenum in fluid communication with the outlet. The tube sheet typically includes a number of apertures and supports a number of filter elements with each filter element covering one of the apertures. Generally, a filter element includes a support structure and a fabric filter media.
In operation, particulate laden or dirty gas is conducted into the baghouse, and more specifically into the dirty gas plenum, through the inlet. The gas then flows through the fabric filter media to the interior space within the filter cores. As the gas flows through the filter media, the particulate matter carried by the gas engages the exterior of the filter media and either accumulates on the filters or falls to the lower portion of the dirty gas plenum. Thereafter, the cleaned gas flows through the apertures in the tube sheet and into the clean gas plenum. The clean gas then flows out of the baghouse through the outlet.
As particulate matter accumulates or cakes on the filters, the flow rate of the gas is reduced and the pressure drop across the filters increases. To restore the desired flow rate, a reverse pressure pulse may be applied to the filters. The reverse pressure pulse expands the filter media and separates the particulate matter, which falls to the lower portion of the dirty gas plenum. While filter material technology has advanced sufficiently to allow a given filter element to be cleaned in this manner tens of thousands of times before replacement is needed, further extension of a filter's useful life is economically desirable. Extended filter life not only saves the cost of filters, it also saves the cost of filter replacement, which is often difficult, costly and requires the baghouse to be taken out of service for a period of time.
Another common technique for separating particulate matter from a gas stream is to use an electrostatic device, such as an electrostatic precipitator. In this device, particulate matter is electronically charged and then collected through the action of an electric field. A typical electrostatic device provides a discharge electrode that is maintained at a high voltage and a non-discharge electrode that is maintained at a relatively lower voltage or at ground. As the particulate-laden gas steam flows past the electrodes, the electric field present between the electrodes operates to charge a percentage of the passing particulate matter and causes them to collect on the non-discharge electrode. Further, when a discharge electrode is used in conjunction with a fabric filter, the charged particles create a dust layer that is less dense because the like-charges of the particles repel each other. With a less dense layer of particles, the period between pulse cleanings may be increased without incurring an increase in the pressure drop across the fabric filter.
Various sorbents may be injected into exhaust or flue gas for the purpose of adsorbing gaseous phase pollutants. The type of sorbent injected generally is a function of the pollutant to be removed. The injection rate of the sorbent generally is a function of the desired pollutant removal rate and the effectiveness of the sorbent. For example, powdered activated carbon (hereinafter “PAC”) is a sorbent that may be injected into flue gas for the purpose of adsorbing the gaseous pollutant mercury. Typically, PAC is injected in quantities ranging from 0.5 to over 10 pounds per million actual cubic feet of flue gas treated. The cost of injecting enough PAC such that adequate exit levels of mercury are achieved per current regulatory standards can be a considerable one.
At present, given the potentially longer period between pulse cleanings, filtration systems generally do not take into account efficiencies that may be realized by manipulating sorbent injection rates, fabric filter cleaning cycles and other operational variables. That is, with the discharge electrode and the resulting longer periods between pulse cleanings, fabric filtration systems may be operated in a more cost effective method, saving cleaning cycles, prolonging filter bag life, and/or reducing the use of sorbents, as described in detail below.